U.S. Pre-Grant Publication No. 2003/0209221, published Nov. 13, 2003 (the '221 publication), discloses a two-dimensional rotary displacement device comprises a housing, an outer rotor and at least one inner rotor. The axes of rotation of the outer rotor and the at least one inner rotor are parallel. The inner rotor rotates within the outer rotor as the outer rotor rotates within the housing. International PCT Publication No. WO07/019703, published Feb. 22, 2007 (the '703 publication), discloses a two-dimensional rotary displacement device comprising a stator with an internal rotor that spins on a shaft on a rotating carrier. As the carrier rotates within the stator, the inner rotor spins around the shaft and meshes with the stator. Each of these devices uses an inner rotor.
U.S. Pre-Grant Publication No. 2007/0295301, published Dec. 27, 2007 (the '301 publication), describes an inner rotor suitable for such applications. There is described below a further improvement on the inner rotor described in the '301 publication.